The present invention relates generally to an optoelectronic assembly, and particularly to a photodetector assembly.
A receiver in an optical communications system incorporates an optical detector. The optical detector converts received optical signals to electrical signals. As the demand for greater transmission rates increases, there is a like demand placed on the speed of the optical detector.
Typical optical detectors have a detecting layer, also referred to as an active or absorber layer, sandwiched between at least two other layers. The active absorber layer may be an intrinsic (I) layer of semiconductor, which is disposed between a p-doped semiconductor layer and an n-doped semiconductor layer. This structure is commonly referred to as a PIN structure, and the detector as a PIN detector (shown generally in FIG. 9). Optical signals may be incident perpendicularly to the top surface of the detector (shown at 901). This type of detector is referred to as a surface receiving detector. Alternatively, optical signals may be incident perpendicularly to the side of the detector (shown at 902). This is referred to as an edge receiving detector. There are advantages and disadvantages to both types of detectors.
In the surface detector, the area of incidence for an incident optical signal is greater than in the edge detector. This is because the length (1) and width (w) are generally proportionately much greater than the thickness (t) (See FIG. 9). Accordingly, the surface device has a greater amount of light incident on the active absorber layer, making optical coupling to the active absorber area more efficient in the surface detector than in the edge detector.
However, the portion of light absorbed by the active area, which recombines to form carriers (referred to as the optical efficiency) depends on the depth the optical signal penetrates into the light absorbing layer. In the surface detector, the penetration depth is limited by the thickness of the active layer.
One way to increase the speed of the detector is by reducing the thickness of the active layer. This reduces the carrier transit time, which is the time it takes a carrier to travel out of the active layer to either the p-type or the n-type layer. The shorter the time required for the carriers to traverse the active layer, the faster the response time. So, a faster transit time results in a faster device. Of course, a thinner active layer limits the depth to which the light can penetrate and thus be absorbed. This may reduce the optical efficiency of the device, making faster surface detectors difficult to realize.
The optical efficiency is intrinsically greater in the edge detector compared to the surface detector. This is because the penetration of light is along the length (l) of the active layer in the conventional structure shown in FIG. 9. This allows a greater penetration depth by the incident light, resulting in a greater degree of absorption. Moreover, the carrier transit time out of the active layer may be reduced by reducing the active layer thickness without diminishing the optical efficiency. Accordingly, due to the orientation of the incident optical signal, the speed of the device may be improved without compromising optical efficiency.
However, there are drawbacks to the edge detector. The edge detector may form a waveguide with the active layer being the guiding layer of the waveguide. Unfortunately, the coupling efficiency of such an edge detector may be poor. The small size of a supported mode (which corresponds to a large far field) in the waveguide of the photodiode makes it difficult to efficiently couple light from some sources, such as optical fibers. Accordingly, only a small portion of the incident light may couple to the edge detector, thus limiting the amount of light absorbed by the detector.
Finally, there are other potential drawbacks associated with edge detecting devices. As is known, device speed may be adversely impacted by junction capacitance. Junction capacitance is directly proportional to the device area. In an edge detector, the area is dependent on the length of the active layer. As such, to reduce the junction capacitance, and thereby improve the device speed, the length of the device may have to be reduced. As a result, the optical efficiency may be compromised in the pursuit of higher device speed. Furthermore, achieving the actual reduction in detector length has its shortcomings. Normally, the device length may be shortened by cutting (or cleaving) the device to a desired length. Unfortunately, cleaving tolerances by present day techniques are often of the same order of magnitude as the desired length of the high-speed device.
Accordingly, what is needed is a high-speed optical detector assembly that has good optical coupling and high internal efficiency.
The present invention relates generally to an optical device for coupling an optical signal to an optical detector. The present invention includes a mode compression section disposed between an optical input source and a photodetector. The mode compression section which enables efficient coupling between the optical input source and the photodetector.